Among the world’s investment nerds, copper has a special title: They call it “Dr. Copper” because of its reputed ability to predict the direction of the global economy as well as any Ph.D.-holding economist might.

Its reputation reflects the metal’s broad use in important, cyclical industries ranging from construction to automobiles. If demand for copper starts to fall off, pushing prices lower, the argument goes, then it’s a good clue that industrial economies are slowing down.

Right now, while American stock investors don’t seem too bothered by the impact of a trade war, the metal is raising a red flag. The price of copper has fallen 19 percent just since early June, and it’s now at its lowest in about a year. That’s a steeper drop than for other commodities like crude oil, which has also been falling in recent weeks.

Concern about China, the single largest consumer of the metal, is the primary source of this weakness. The country’s economic outlook has darkened since the trade war with the United States began, right around when copper prices were at their peak.

“The sell-off started with the tariffs,” said Natasha Kaneva, an analyst who follows copper at JPMorgan Chase in New York. “Absolutely you can say that that was the catalyst.”

The trade war is hitting China as it also faces a difficult transition away from the growth model that fueled the country’s rise — a mix of debt, state-directed investment and export-focused factory activity — toward domestic consumption. Recent economic data suggests that it is not a smooth process.

In the second quarter, nonperforming loans at Chinese banks notched their biggest rise in over a decade, according to research from Capital Economics. Corporate bond defaults are on the rise, and earlier this week, official reports showed Chinese investment growth, which has long been a driver of the economy, fell to its lowest level since the late 1990s.

“There is a combination of things going on,” said Simona Gambarini, a commodities economist at Capital Economics. “A strong dollar, China slowing, concern about a trade war between China and the U.S., which doesn’t bode well for China in particular but for the global economy as well.”

The pressure on the Chinese economy is beginning to take its toll in other places, too. China’s currency, the renminbi, has fallen more than 9 percent against the dollar in the last six months and China’s CSI 300 index of blue chip stocks is off 19 percent this year.

Such concerning signals emanating from a country that accounts for roughly 15 percent of the world economy might give some investors pause. But for now, investors in the United States seem to see China as someone else’s problem.

The Standard & Poor’s 500-stock index is up more than six percent this year and continues to hover just below its record high, thanks to the health of the American economy. That growth — plus the windfall of deep tax cuts — has translated into gushers of corporate profits. Second-quarter earnings reports from companies in the S.&P. 500 are up roughly 25 percent from last year.

But if the slump in copper prices is a harbinger of a significant slowdown in global economic growth, American investors could eventually feel the pain.

Large American companies rely on sales outside of the United States for substantial portions of their revenue. Last year, 44 percent of the revenues of companies in the S.&P. 500 came from foreign countries, with Asia being the single largest regional source of sales, according to data from S.&P. Dow Jones Indices.

Investors seem broadly split over whether American markets can continue to separate themselves from worries about global growth.

In early August, Bank of America Merrill Lynch surveyed 185 money managers with over $500 billion in assets. One question asked whether the American economy could continue outperform the rest of the world.

Respondents were split, with 34 percent saying they thought the United States could continue to do better and 32 percent saying they expected growth in the United States to start to slow down.

The majority of the fund managers, however, agreed about what could be the biggest risk to the market. About 57 percent answered that the top risk was “trade war.”

A version of this article appears in print on , on Page B1 of the New York edition with the headline: Cooling Copper Could Signal Slowdown. Order Reprints | Today’s Paper | Subscribe